Next week the Royal Ballet will be dancing Romeo and Juliet for the 420th time – which, if you do the maths, means that Kenneth MacMillan's ballet has averaged nearly 10 performances a year since its premiere in 1965. Few works can survive such exposure, and there are parts of it I'd happily never see again. But I'll still be there for Tuesday's show, because, like all of MacMillan's best work, the ballet operates at a level of emotional and psychological complexity that nearly always challenges its dancers to reveal something new.

Much has been said about the MacMillan legacy in this, his 80th anniversary year. But what interests me is the impact he's made on the acting style of the Royal Ballet, his home company. Famously, MacMillan's ballets occupy a dramatic terrain that's darker, more elusive and more contradictory than that of the classical repertory – which means that any dancer performing his work has to learn to act outside the standard lexicon. To portray Manon, a ballerina has to show love and concupiscence, naivety and worldliness; while Rudolf, in Mayerling, has to be seen to unravel through lust, madness, betrayal and dread.

MacMillan's choreography gives the dancers lots to work with, his physical language ranging from the twitch of a shoulder or the blanking of a glance, to full-bodied extremes of sex, grief and joy. But it also requires dancers to think and feel for themselves. I'm sure the reach and ambition of Royal performers derives from the fact that they do so much MacMillan. On top form, this company is the RSC of the ballet world, and their acting skills infect the rest of the repertory.

Inevitably, the standard of dancing doesn't always measure up to the acting; in ballet, it's rare to get technique and expression at equal pitch. But sometimes, it happens – and, given that Tuesday's Juliet will be Tamara Rojo, I'll happily tolerate the tedium of the sword fights and processionals for the fire and steel of her performance.